President Barack Obama and Republican challenger Mitt Romney don't agree on much but they do on at least one thing: they're both fond of their respective running mates.

Romney and Rep. Paul Ryan of Wisconsin teamed up again Friday — the second time this week — for a pre-convention rally on a farm in Commerce, Mich., a Detroit suburb. The two seem comfortable with each other in public and Ryan, 42, seems to energize the man topping the ticket who is 23 years his senior.

"What a guy. What a vice president he's going to be," Romney told a crowd of thousands after Ryan introduced him with high praise.

Obama also had nice things to say about his running mate. Four years have now passed since he chose Biden for his ticket, Obama tweeted Thursday night. "Here's to an outstanding Vice President and a great friend -bo." Obama ends his personal tweets with his initials.

Biden caused a stir earlier this month when he told a Virginia crowd that included hundreds of black people that Romney's plans for Wall Street would put them "back in chains."

Republicans have condemned the phrase, suggesting it was racist. Biden has countered that, while the choice of words may have been unfortunate, it was just a variation on his accusation that Romney and his GOP allies would "unshackle" banks and other financial institutions from federal regulation.

Obama was taking a short break from the campaign trail and spending the weekend at the presidential retreat at Camp David, Md.

The four-day GOP convention opens Monday in Tampa, Fla. Now moved up to the first day: the formal state-by-state roll call of delegates that will pave the way for Romney's nomination as the party's presidential candidate.